Figure 5.

Hypothetical scenario of the evolution of bicoid and zerknüllt. The common ancestral gene is thought to have been expressed maternally (hatched) providing transcripts throughout the early embryo, and zygotically (filled) in the extraembryonic tissue anlage. After duplication, presumably in the stem lineage of Cyclorrhaphan flies, zygotic expression is lost in the bicoid daughter and maternal expression is lost in the zerknüllt daughter. Opposing functions of bicoid and zerknüllt (see text) evolved after the spatial separation of expression domains, caused by the anterior localization of bicoid and the reduction of the extraembryonic anlage/tissue. a, anterior; p, posterior.